1. Field of the Invention
The present invention relates to gas discharge display and memory devices and methods therefor. More particularly, the present invention relates to AC gas discharge display memory devices utilizing improved writing and erasing techniques.
2. Description of the Prior Art
Gas discharge display and memory panels of the type to which the present invention appertains are well-known in the prior art. For example, U.S. Pat. No. 3,499,167 to Baker et al describes such a display and memory panel. The gas panels of the type of which the present invention is directed typically utilize two glass plates, maintained in spaced-apart relationship, and are arranged to have sealed between the spaced-apart plates an ionizable gaseous medium. To provide matrix addressability whereby selected local regions within the ionizable medium may be selectively ionized, sets of horizontal and vertical conductors are employed. Typically, the set of horizontal conductors comprises an array of parallel insulated conductors arranged on the inner surface of one plate and horizontally extending thereacross. Likewise, the set of vertical conductors comprises an array of parallel insulated conductors arranged on the inner surface of the other plate vertically extending thereacross, generally orthogonal to the horizontal conductors.
In such an arrangement, when an appropriate voltage is applied between a selected one of the horizontal conductors and a selected one of the vertical conductors, ionization occurs at the cross-over point of the two conductors such that light is emitted. Generally, the cross-over points are referred to as "cells," and a display pattern or image is formed by ionizing selected cells. As another example of a panel as described and to which the present invention pertains, reference is made to an article by D. L. Bitzer et al entitled, "The Plasma Display Panel -- A Digitally Addressable Display with Inherent Memory". Proceedings of the Fall Joint Computer Conference IEEE, San Francisco, Calif., November 1966, pp. 541-547.
One of the difficulties with conventional AC plasma display panels, as they exist today, resides in the fact that tight voltage margins in the erase and write operations result in high panel cost and reduced panel yield. As generally operated today, AC plasma display panels respond quite differently to write and erase operations. In general, errors in write pulse amplitudes due to cell and circuit margins produce errors in the "on-state" wall voltage (compared to the stable steady state value of wall voltage) which disappear within a very few sustain cycles because the wall voltage gravitates toward the stable point. On the other hand, errors in erase pulse amplitudes can produce "off-states" with improper wall voltages which persist for long times due to the lack of strong discharge activity in the off state. Even when this does not result in erase failures, it can reduce margins by causing the cells to respond incorrectly to subsequent write pulses and half-select pulses. As a consequence of this difference between write and erase operations, pulses of quite different wave shape are presently used for writing and erasing even though the voltage to be transferred, and so the intensity of the required discharge, is the same in the two cases.
In accordance with the principles of the present invention, an alternative writing and erasing scheme is proposed wherein the erase operation is followed and/or the write operation is preceded by strong discharge sequences which bring the wall voltage of each cell to the proper stable value for that cell. Although it is known in prior art gaseous discharge display panels to electronically condition the gaseous discharge by periodically causing unstable discharges at sites which are not in the "on" state, as taught for example in U.S. Pat. No. 3,559,190 or in U.S. Pat. No. 3,833,831, it is not known in the prior art to "normalize" the state of cells before writing and after erasing, with waveforms as taught in accordance with the present invention.